This invention relates to thermoplastic polyurethanes. Particularly, although not exclusively, the invention relates to a method of manufacturing thermoplastic polyurethanes.
Thermoplastic polyurethanes (TPUs) based on the reaction of polyisocyanates with polymeric diols are well-known for use as elastomers, adhesives, sealants, elastomeric surface coatings, and coatings for metals and plastics. As will be described in detail hereinafter, the polyurethanes of the present invention are based on a polydiene diol, an isocyanate, and a relatively low molecular weight chain extending diol. In this composition the polydiene serves as the soft, rubbery portion of the polyurethane. The diisocyanate and chain extending diol react together to form a rigid segment of the polymer. Because of the reactivity of isocyanates and alcohols, the polydiene is chemically bound at its termini to the hard segments. In this way a segmented block copolymer is formed.
The strength and other physical properties derive from the molecular structure of the segmented block copolymer polyurethane. In their high molecular weight polymerized form, the soft and hard segments are significantly incompatible and segregate into separate domains. In this arrangement the soft segments act as rubbery polymers and they are physically crosslinked through the segregation and vitrification or crystallization of the hard segments. It is important that the incompatibility between hard and soft segments exists in the final high molecular weight form. Without this incompatibility a leathery material would result which would have limited useful applications due to poor low temperature properties and a low upper service temperature.
There are various known methods of manufacturing TPUs. This invention is based on the discovery that TPUs having desirable but unexpected properties can be made using a particular method of manufacture.
According to a first aspect of the invention, there is provided a method of manufacturing a thermoplastic polyurethane (TPU) from:
(i) a hydrogenated polydiene diol having from 1.6 to about 2 terminal hydroxyl groups per molecule and a number average weight of at least 500 and less then 20,000; and
(ii) an isocyanate having two isocyanate groups per molecule; and
(iii) one or more chain extenders; and
(iv) optionally a catalyst
the method using a co-rotating reaction extruder.
It has surprisingly been found that the method can produce TPUs having unexpectedly advantageous properties. For example, TPUs prepared in the method may have tensile strengths of greater than 20 MPa. Although tensile strengths greater than 20 MPa are common for TPUs based on polar polyols such as polyesters and polyethers, it was believed to be impossible to obtain such high strengths from TPUs based on hydrocarbon polyols.
Said reaction extruder is preferably a multiple screw reaction extruder. Said method using said extruder preferably does not include any solvent. More preferably, the reaction of said isocyanate, said polydiene diol and said chain extender is solvent free.